The present invention generally relates to a nonreciprocal circuit element of a lumped constant type such as an isolator or a circulator which are employed in high-frequency components having a frequency band in the VHF, UHF and microwave ranges and more particularly, to a nonreciprocal circuit element which can be made compact without increasing its production cost. Since the present invention is preferably applicable to an isolator employed in, for example, a mobile telephone system, a prior art isolator in a mobile telephone system will be described as one example, hereinbelow.
The mobile telephone system of this example is a mobile telephone system which can carry out transmission and reception in the same manner as general fixed telephone sets by using radio waves in a band ranging, for example, from 800 to 900 MHZ. As shown in FIG. 1, this mobile telephone system is constituted by mobile telephone equipment generally designated 50, an antenna 51 used for both transmitting signals and receiving signals in common, and a telephone set 52. The mobile telephone equipment 50 includes a transmitter 53, a receiver 54, a controller 55 and a duplexer 56. The controller 55 is provided for giving commands for effecting transmission between the mobile telephone system and base stations, changeover of channels, etc. The duplexer 56 is provided not only for preventing interference between the transmitted signals and the received signals but also for preventing interference signals, from being emitted externally. An isolator 57 for preventing reflection of transmitted RF power is provided between the transmitter 53 and the duplexer 56. The function of the isolator 57 is to pass signals with very slight attenuation in a direction from the transmitter to the duplexer but to greatly attenuate signals in the opposite direction and is an indispensable component for the mobile telephone system.
The known isolator 57 has a construction as shown in, for example, FIGS. 2 and 3. In FIGS. 2 and 3, an isolator 30 includes a metallic casing 31 acting as an outer conductor and having a shape of a rectangular parallelepiped, an earth plate 32 made of copper, and a substrate 33 made of alumina. The substrate 33 is placed on the earth plate 32 which in turn is on the bottom of the casing 31. The substrate 33 is formed, at its central portion, with a hole 33A. A ferrite assembly 34 is inserted into the hole 33A. A permanent magnet 35 is bonded to an inner face of an upper wall of the casing 31.
The ferrite assembly 34 includes a pair of upper and lower ferrite members 34a and 34b. Central conductors 37a, 37b and 37c are provided between the upper and lower ferrite members 34a and 34b so as to intersect with one another at an angle of 120.degree. and such that the central conductors 37c and 37a confront the upper and lower ferrite members 34a and 34b, respectively. Furthermore, two insulating sheets 36 are, respectively, inserted between the central conductors 37a and 37b and between the conductors 37b and 37c. An earth piece 37d is integrally formed with the central conductors 37a, 37b and 37c. A bottom face of the lower ferrite member 34b is connected, through the earth piece 37d, to the earth plate 32. Distal ends (lead-out portions) of the central conductors 37a, 37b and 37c are, respectively, connected to capacitor electrodes 38a, 38b and 38c formed on peripheral portions of an upper face of the substrate 33. The capacitor electrodes 38a, 38b and 38c act as elements for impedance matching. A contact piece 37e extends upward from each of the central conductors 37a, 37b and 37c and is fitted into each of the holes of an earth plate 39 provided on an upper face of the upper ferrite member 34a so that all three contact pieces 37e are connected to the earth plate 39. Thus, the earth plate 39 is connected to the earth plate 32 so as to assume earth potential. Meanwhile, the capacitor electrode 38c is connected, via a film resistor 40, to an earthing electrode 38d for the substrate 33 by a through-hole electrode 41. The remaining capacitor electrodes 38a and 38b are led outwardly by external terminals 42, respectively.
FIG. 4 shows an equivalent circuit of the isolator 30. For example, a signal inputted to a terminal A is passed through only a terminal B, while a signal flowing in a direction from the terminal B towards the central conductor is absorbed, through its conversion into heat, by the film resistor 40.
Since the mobile telephone equipment, the telephone set, etc. are required to be loaded into a small cabin, there is a keen demand that the mobile telephone system be made as compact as possible. In accordance with this demand for compactness of the mobile telephone system, there is a demand that the isolator be also made more compact. As a way to meet this demand for compactness of the mobile telephone system, a possible reduction in the diameter of, for example, the ferrite members has been considered. However, if this is done, electrical characteristics of the isolator are aggravated. Therefore, it is not desirable for the ferrite members to be made smaller in diameter.